Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by application of a transparent or clear topcoat over the base coat have become increasingly popular as original finishes for automobiles. The color-plus-clear systems have outstanding appearance properties such as gloss and distinctness of image, due in large part to the clear coat.
The most economically attractive color-plus-clear systems are those in which the clear coat composition can be applied directly over the uncured colored base coat. The process of applying one layer of a coating before the previous layer is cured, then simultaneously curing both layers, is referred to as a wet-on-wet ("WOW") application. Color-plus-clear coating systems suitable for WOW application provide a substantial energy cost savings advantage.
Over the past decade, there has been an effort to reduce atmospheric pollution caused by volatile solvents which are emitted during the painting process. However, it is often difficult to achieve high quality, smooth coating finishes, particularly clear coating finishes, such as are required in the automotive industry, without including organic solvents which contribute greatly to flow and leveling of a coating. In addition to achieving near-flawless appearance, automotive coatings must be durable and chip resistant, yet economical and easy to apply.
The use of powder coatings to eliminate the emission of volatile solvents during the painting process has become increasingly attractive.
Powder coatings have become quite popular for use in coatings for automotive components, for example, wheels, axle parts, seat frames and the like. Use of powder coatings for clear coats in color-plus-clear systems, however, is somewhat less prevalent for several reasons. First, powder coatings require a different application technology than conventional liquid coating compositions and, thus, require expensive modifications to application lines. Also, the high standard of automotive clear coats is, for the most part, set by polyurethane systems, which are typically cured at temperatures below 140.degree. C. Most powder coating formulations require a much higher curing temperature. Further, many powder coating compositions tend to yellow more readily than conventional liquid clear coating compositions, and powder clear coating compositions generally result in clear coatings having a high cured film thickness, typically ranging from 60 to 70 microns.
U.S. Pat. No. 5,379,947 discloses a process for producing a powder coating composition wherein the powder particle size does not exceed 100 micrometers and at least 50 percent of the powder particles are of a size ranging from 3 to 5 micrometers. The powder coating compositions can include any of a variety of polymeric resins including acrylic, epoxy, amine-modified, phenolic, saturated or unsaturated polyester, urea, urethane and blocked isocyanate resins, or mixtures thereof. After milling, the powder is added to a mixture of water and surfactants, followed by the subsequent addition of dispersants and rheology control agents, thereby forming a powder slurry. The powder slurry coating compositions are useful for both base coat and clear coat applications.
Powder in slurry form for automotive clear coatings can overcome many of the disadvantages of dry powder coatings, however, powder slurry compositions often tend to be unstable and settle upon storage at temperatures above 20.degree. C. Further, WOW application of powder slurry clear coating compositions over conventional base coats can result in mudcracking of the system upon curing. See Aktueller Status bei der Pulverlackentwicklung fur die Automobilindustrie am Beispiel fuller und Klarlack, presented by Dr. W. Kries at the 1st International Conference of Car-Body Powder Coatings, Berlin, Germany, June 22-23, 1998, reprinted in Focus on Powder Coatings, The Royal Society of Chemistry, 2-8, September 1998.
Generally, any film that contains a volatile component such as water undergoes a decrease in volume as the volatile component evaporates from the surface of the film. As the volatile component leaves the film, contraction forces act to pull the film inward in all directions. However, without intending to be bound by any theory, it is believed that if the film has sufficient cohesive strength, the film will contract in only one dimension, that is, the film thickness will decrease, while the film resists contraction in any direction parallel to the substrate surface. By contrast thereto, if a film lacks sufficient cohesive strength to resist contraction parallel to the substrate surface, contraction forces will cause the film to break up into small flat segments that are separated by continuous linear voids. This surface defect is commonly referred to as "mudcracking".
An aqueous coating that forms a powder upon application at ambient temperature cannot coalesce readily to form a generally continuous film until subjected to thermal cure conditions. The tendency of such coatings to form mudcracks upon curing is believed to be due to lack of sufficient cohesive strength which results from the lack of coalescence of the powder particles prior to thermal curing.
Canadian Patent Application No. 2,203,868 discloses a process for preparing aqueous dispersions which form powder coatings at ambient temperature, which, after curing, provide coatings having improved water and solvent resistance. The dispersions are comprised of a polyol component having a Tg of greater than 30.degree. C. which may be hydrophilically modified, and a blocked isocyanate crosslinker which may be hydrophilically modified. Although applied as conventional waterborne coating compositions, these dispersions form powder coatings at ambient temperature which require a ramped bake prior to undergoing conventional curing conditions in order to effect a coalesced and continuous film on the substrate surface.
U.S. Pat. No. 5,071,904 discloses a waterborne coating composition which comprises a dispersion of polymeric microparticles in an aqueous medium. The microparticles contain a substantially hydrophobic polymer which is essentially free of repeating acrylic or vinyl units in the backbone and is adapted to be chemically bound into the cured coating composition. The disclosed microparticles do not comprise a hydrophobic crosslinker, that is, a crosslinker such as a fully butylated melamine, which is not soluble or dispersible in water. Moreover, the coating compositions, while waterborne, typically contain a substantial amount of organic solvent to provide flow and coalescence to the applied coating.
The automotive industry would derive a significant economic benefit from an essentially solvent-free clear coating composition which meets the stringent automotive appearance and performance requirements, and which can be applied by conventional application means over an uncured pigmented base coating composition (i.e., via WOW application) to form a generally continuous film at ambient temperature which provides a cured film free of mudcracking.